waf: shuffle around some libav checks to have options added
waf: make iconv check mandatory
waf: add filter_and_parse_features to our feature parser

This function will filter dependencies based on the fact that their names
starts with '--', strip the starting '--' and add them as features.

This allows to specify a common list of features / dependencies and doesn't
add yet another property to our dependency hash.
squash me
tmp audio sources
update waf
waf: force use of `cc` in the correct way
waf: add some compiler cflags
waf: audio stuff tmp
waf: add some of the windows checks
waf: add an inflector to generate names of stuff